1. Field of the Invention
The present invention relates to an internal power source voltage generating circuit of a semiconductor memory and a method for generating an internal power source voltage to generate an internal power source voltage so as to drive a semiconductor memory based on a power source voltage externally supplied.
2. Description of the Related Art
A flash memory as a semiconductor memory comprises memory cells arranged in a matrix, comprising EEPROM (electrically erasable programmable read only memory) capable of electrically writing and deleting data. Each memory cell comprises a stack gate transistor having a floating gate FG and a control gate CG, for example (for example, refer to FIG. 25 of Patent Literature 1). Each memory cell is capable of writing and reading data by changing a threshold voltage by means of electron injection or electron release at a floating gate FG. For example, a power source voltage supplied externally (hereafter, referred to as an external power source voltage) is applied to a control gate CG of the memory cell to read data, wherein it is determined if the read data corresponds to logical level 0 or logical level 1 depending on whether electricity flows or not.
According to a conventional flash memory, a control gate voltage to read data is set to 5 V similarly to the external power source voltage, however, as memory cells have become miniaturized and memory capacities have increased, the external power source voltages have decreased. For example, an external power source voltage of 3 V has become general.
A semiconductor integrated circuit was proposed in which a 3 V external power source voltage is boosted to 5 V in a semiconductor chip, for example, which is applied to a control gate CG of a memory cell as an internal power source voltage (for example, refer to FIG. 1 of Patent Literature 1). The semiconductor integrated circuit is provided with a boosting circuit to boost the external power source voltage so as to generate a boosted voltage, a level detecting circuit to generate a signal indicating whether the boosted voltage is lower than a reference value or not, and an internal power source voltage generating circuit to generate a voltage as an internal power source voltage in which the boosted voltage is lowered. The boosting circuit is equipped with a charge pump and oscillator (for example, refer to FIG. 2 and FIG. 3 of Patent Literature 1). The oscillator supplies an oscillation signal obtained by performing an oscillating operation to the charge pump only if the boosted voltage generated by the boosting circuit is lower than a reference value. If the boosted voltage is higher than the reference value, the oscillator stops the oscillating operation so as to supply a prescribed constant value to the charge pump. Only when the oscillation signal is supplied from the oscillator, the charge pump sequentially transfers an electric charge corresponding to the oscillation signal to each of a plurality of condensers, thereby generating a boosted voltage that is higher than the external power source voltage. According to the structure described above, the boosting circuit operates the charge pump when the boosted voltage generated by the boosting circuit itself does not reach the reference voltage, thereby raising the voltage value, whereas it stops the operation of the charge pump when the boosted voltage exceeds the reference voltage, thereby lowering the voltage value. Accordingly, the boosting circuit generates the boosted voltage in which the external power source voltage is boosted by a desired voltage value.
According to the above boosting circuit, it becomes possible to generate the boosted voltage that is higher than the external power source voltage, however, as the voltage value is increased, the amount of the current to supply decreases. In contrast, the consumption current of the memory cell increases as the address period shortens in order to access (read or write the data). Accordingly, in order to use the boosted voltage as the power source voltage of the memory, as the change period of the address is shortened, the boosted voltage value needed to be lowered, hence a minimum address period with guaranteed access was prescribed.
However, dummy reading that does not require with guaranteed access sometimes reads the data in an address period shorter than a prescribed minimum address period so as to shorten the read time. At this time, there has been a possibility that a following operational malfunction might occur immediately after dummy reading was changed to normal data reading.
That is to say, if dummy reading is started in a period shorter than the prescribed minimum period with guaranteed access, as illustrated in FIG. 1, as the consumption current increases due to shortening of the address period, it becomes impossible that a boosted voltage Vbst generated in the boosting circuit keeps a prescribed voltage value Va, which gradually decreases. Thereafter, when the dummy reading operation is changed to a normal data reading operation at a time TQ as illustrated in FIG. 1, that is to say, when it is changed to a data reading operation in the prescribed minimum period, the boosted voltage Vbst generated in the boosting circuit gradually rises to reach the prescribed voltage value Va as the consumption current decreases due to a lengthened address period.
Therefore, even if dummy reading is changed to the normal data reading operation at the time TQ, immediately thereafter the voltage value of the boosted voltage Vbst generated in the boosting circuit has not reached the prescribed voltage value Va so as to read the data normally. Hence there has been a problem in that immediately after dummy reading is changed to the normal data reading operation, normal data reading is not performed until the voltage value of the boosted voltage Vbst reaches the prescribed voltage value Va, causing an access delay to occur.